The next time you book a flight, you'll want to be very sure about your plans. Especially if that flight is on US Airways or United.

Last week, United quietly increased the fee for changing a flight from $150 to $200 for domestic flights. Today, US Airways matched the fee increase. The change was reflected on their web site.

The $150 change fee has become industry standard and is charged by the majority of domestic carriers. One notable exception: Southwest, which does not charge a fee to change flights.

Both United and US Airways now charge more -- between $250 and $300 -- for flight changes to Europe and Asia.

"We carefully manage our seat inventory and incur costs when a traveler elects not to fly in a reserved seat," United spokeswoman Jennifer Dohm told ABC News. "We adjusted this fee to better compensate us for those costs."

US Airways could not immediately be reached for comment.

It's leisure travelers that are most likely to have to pay the increased fee. Leisure travelers are, for the most part, traveling on the economy-class, least-expensive, non-refundable tickets that are subject to change fees. Refundable, flexible tickets, as well as business and first-class tickets, are most often sold to business travelers at a higher price. With that higher price comes the luxury of changing plans – without incurring any extra fees.

The match of United's fee increase by US Airways is telling. If history is any indicator, the airlines' competitors will wait and watch for any significant backlash from the traveling public. If there is none, they may too then decide to increase their fee.

Change fees are in addition to the charge for the difference in fare. So, if a traveler were to purchase a domestic ticket for $300 and then want to change the dates of travel, and the price had then increased to $350, the airline would charge the fare difference of $50 plus the change fee. On United and US Airways, this example would run the customer an additional $250 total.

Like we said, best to be sure about your plans before booking an airline ticket.